Interaxle differentials distribute drive power from an onboard power source (e.g., vehicle's engine) between two drive axles (front drive axle and rear drive axle). An input drive shaft conveys engine power to the transfer case, and front and rear output drive shafts convey separate portions of the engine power en route to the front and rear axles.
The power division between output drive shafts is accomplished by a differential gear unit. Unbalanced speed ratios across the differential are provided for imbalancing torque divisions between the output drive shafts. In addition, transfer cases can be arranged to provide a choice of gear ratios between the input drive shaft and the two output drive shafts collectively. Planetary gear units are typically used for this purpose. The planetary gear units can be shifted between positions supporting the different drive gear ratios (typically referred to as high and low ranges).
In a typical configuration, the input drive shaft to the transfer case and the two output drive shafts from the transfer case are aligned with the same axis of rotation. The input drive shaft is typically coupled directly to the transmission, and the rear output drive shaft is typically coupled directly to a rear propshaft. The front propshaft to the front drive axle is typically offset from the common axis of the input and output drive shafts of the transfer case and is connected to the front output drive shaft by a sprocket and chain or other form of parallel-axis drive mechanism.
The multiple gear units and concentric shafts provide challenges for mounting and supporting the transfer case components. For example, in arrangements in which the front and rear output drive shafts emerge together from the gear units, the rear output drive shaft is received in a bearing of the transfer case housing and provides an outboard support for both gear units within the transfer case. In addition, the rear output drive shaft typically provides support for the bending forces generated by the sprocket and chain to the front output. The multiple functions of supporting the interior gear units and parallel-axis drive mechanism require the rear output drive shaft to be dimensioned larger than would otherwise be required for its primary role of conveying a portion of the engine power toward the rear drive axle. The enlarged dimensioning of the rear output drive shaft has the effect of enlarging the overall size of both the planetary gear unit and the differential gear unit within the transfer case. The additional sizing requirements also add weight and cost to the transfer case.
In other configurations, the front output drive shaft emerges from the transfer case in the opposite direction concentric with the input drive shaft. Power take-offs from the front end of the transfer case, however, have the effect of shortening the propshaft to the front drive axle. The front propshaft connects to the front drive axle through a front differential. Any unintended offsets between the front differential and the propshaft's connection to the transfer case produce angular errors in the mounting of the propshaft. The angular errors, which can be particularly detrimental to the smooth transfer of power, are amplified by shorter propshafts. Thus, transfer case arrangements that connect the front output drive shaft of the transfer case to the front propshaft at the front end of the transfer case are subject to undesirable angular misalignments.
Other difficulties arise because of particular gearing arrangements within the planetary and differential gear units. For example, single-pinion differential gear units in which individual pinion gears interconnect internal and external side gears pose special design challenges, particularly for directing front and rear drive shafts from a common end of the transfer case.